Yes, water has a very high specific heat capacity compared to most other materials. Having a water tank inside the greenhouse works well, either a dip tank, or a butt fed from the gutters. Or a couple of full watering cans ready for use (heat capacity, and warmed water ready for watering). If building new greenhouse staging, one could use salvaged radiators. I've seen photos by Bob Flowerdew using radiators laying flat as paths in a polytunnel.
I use bricks outdoors too- under aubergine plants, and inside cloches.